Many comminuted materials are conveniently transported by truck. Typically, such trucks include a tractor and an attached trailer having a tank or other container mounted thereon. Frequently, these trailers are referred to as pneumatic tank trailers because of the pneumatic method by which they are typically loaded and unloaded. Conventionally, comminuted materials are transferred from a storage unit to a container such as a trailer-mounted tank by pressurizing the storage unit with air, connecting a conduit from the storage unit to the tank, and permitting the product entrained in the air flow to pass through the conduit from the high-pressure storage unit to the lower-pressure tank. Although the conventional method works well with many types of comminuted materials, problems of product degradation may arise if the materials are temperature sensitive, and there may be a significant risk of explosion if the product is combustible. Because an increase in pressure on a gas is directly proportional to an increase in its temperature (so long as its volume remains constant), pressurizing the air in the storage unit will increase its temperature, and this increased temperature will be transmitted to the product. If the product is temperature sensitive, such as plastic pellets, for example, pressurizing the air in the storage unit may even raise the temperature of the product past its melting point. In addition, the transport by air entrainment of comminuted product may result in the generation of sparks by friction between the product and the discharge port or hopper of the storage unit or the conduit within which the product is transported, or in the generation of sparks resulting from static electricity charges that are produced in the atmosphere of the tank as the product is transported into it. If such product is finely divided and combustible, the generation of such sparks may result in explosion.
Various methods have been developed to deal with the problem of transport of temperature sensitive or combustible products. These methods generally involve a cooling of the pressurized air or a substitution of an inert gas for the pressurizing air. Thus, for example, it is known to employ a water-filled sheath around the conduit carrying the pressurizing air stream to the storage unit. However, such a device requires that a supply of water be readily available at the point of transfer, as well as the means for its disposal. It is also known to place cooling fins on the product conveying conduit. However, such cooling fins are bulky, heavy and expensive, and only marginally effective. It is also known to reduce the temperature created in a pressurizing system by restricting the operation of the compressor to reduce the pressure produced. However, such restriction will slow the transfer process since the pressure differential between the high-pressure storage unit and the tank will not be as great. U.S. Pat. No. 5,580,193 describes another method for reducing the temperature of the pressurizing air by placing a heat exchanger in the air stream. However, such apparatus, which may be effective in reducing the temperature to which the product is exposed, will not reduce the risk of explosion due to sparking in the transport of finely divided combustible product.
It is also known that nitrogen may be substituted for air as the pressurizing gas in order to reduce the risk of explosion during transfer of a combustible product. Pumping additional nitrogen into the storage unit may be employed to reduce the oxygen content of the air in the unit below that which would support combustion. However, a nitrogen pressurizing system will increase the temperature of the resulting gas mixture as much as an air pressurizing system. In addition, a nitrogen pressurizing system requires that a nitrogen source be available at the product transfer point, and it requires a relatively large volume of nitrogen to be effective. For example, a nitrogen pressurizing system that is aimed at reducing the oxygen content by half (from about 21% to about 10.5%) in a 1500 cubic foot storage unit will require the introduction of 1500 cubic feet of nitrogen. Thus, the requirement for provision of a nitrogen source limits the locations at which the product may be transferred to facilities having sufficient quantities of such gas available, and the use of nitrogen increases the expense of transferring the product.
It is also known that a conduit may be connected between a storage unit (such as a railcar) that is open to the atmosphere and a trailer mounted tank, and the air in the tank withdrawn and evacuated to the atmosphere so as to reduce the pressure in the tank below that in the storage unit. Comminuted product stored in the storage unit will then flow under the influence of the higher atmospheric pressure in the railcar to the lower pressure tank. Although this type of loading system will not likely result in a significant increase in the temperature of the product being transferred, it will entail a risk of explosion due to sparking in the transport of finely divided combustible product.
It would be desirable, therefore, if a method and apparatus could be provided that could be used to transfer comminuted product from a storage facility to a container such as a trailer-mounted tank while minimizing any increase in the temperature to which the product is exposed. It would also be desirable if such a method could be provided that would minimize the risk of explosion during transfer of the comminuted materials. In addition, it would be desirable if such a method could be provided that would not require that special facilities be available at the transfer point. It would also be desirable if an improved trailer-mounted tank could be provided that would operate according to such method at conventionally equipped transfer facilities.